In oilfield service work, a piece of equipment referred to in the trade as a "tree saver" or wellhead isolation tool is often used. This apparatus generally introduces a high pressure hollow mandrel with a sealing nipple through the low pressure rated valves and fittings on a wellhead and the resilient sealing material on the nipple at the lower end of the mandrel seals the mandrel in the well casing or tubing. This allows high pressure fluids to be introduced into the well casing or tubing through the mandrel without their having any communication with these low pressure valves and fittings. For the remainder of this application, the word casing will be used for casing and tubing. There is an existing wellhead isolation tool designed and manufactured by McEvoy circa 1955 which is still utilized in well servicing with good results considering the changes in well servicing technology since its inception. However, problems with the sealing of the nipples in large diameter casing due to the diametral tolerances in these casings have been prevalent. The upgrading of this particular apparatus with the concentric mandrel and expanding nipple system described in both my Canadian Pat. No. 1,217,128 issued Jan. 27, 1987 and my U.S. Pat. No. 4,657,075, issued Apr. 14, 1987 is proposed.